This gravimetric detection generally relies on the detection of variations in the vibration frequency of an electromechanical oscillator when a molecule is deposited on its surface.
Any oscillator subjected to a vibration force in turn enters into vibration. At certain frequencies, specific to each oscillator, the vibration amplitude is maximal. These frequencies are called “resonance frequencies”. Thus, subjecting an oscillator to a vibration of a frequency equal to one of its resonance frequencies makes it possible to detect the deposition of a particle on the oscillator. In practice, this deposition modifies the vibration frequency of the oscillator which therefore no longer vibrates at a resonance frequency. The vibration amplitude is then reduced and can easily be detected. The minimum amplitude difference that can be detected determines the resolution of the detection device, in other words the minimum detectable weight of a particle being deposited on the oscillator.
This method can be used to:                detect the presence of a molecule,        detect the weight of the molecule,        characterize the kinetics of association/dissociation of a molecule with a complementary molecule recognition element (antibodies, nucleic acid probes or even printed polymer) previously intentionally grafted onto the surface of said oscillator.        
This invention therefore relates to the field of methods for the direct detection, without marking, of biological or chemical events.
Many documents describe gravimetric detection sensors based on NEMS/MEMS oscillators.
Thus, it is possible to cite works in which use is made of a mass sensor in the form of a fixed-fixed beam made of silicon carbide, made to vibrate by magnetostatic transduction.
Throughout the description, the term “beam” will be understood to mean an elongate part, substantially horizontal, the thickness and the width of which are of the same order of magnitude (ratio between approximately 0.8 and 1.2), which are relatively low relative to the length of the beam.
This sensor has made it possible to detect and weigh xenon atoms, deposited in a high vacuum (10−10 torr) and at very low temperature (4.2° K), on the surface of the sensor. Such a sensor exhibits a resolution of the order of seven zeptograms (zg) and a mass sensitivity of the order of 0.96 Hz/zg.
The highly restrictive conditions of operation of this sensor make it difficult, or even impossible, to use for biological purposes because the magnetostatic transduction is low in an aqueous medium and the powerful vacuum is incompatible with in vivo measurements.
Also known are a number of works regarding gravimetric detection with electromechanical oscillators working in an aqueous medium. Thus, it is possible to cite the article by T. Adrega et al., “Resonance of electrostatically actuated thin-film amorphous silicon microelectromechanical systems microresonators in aqueous solutions: Effect of solution conductivity and viscosity”, J. Appl. Phys.2007, 101, 094308. This article describes a fixed-fixed beam set to vibrate off-plane, and the resonance frequency of which is measured through an optical detection. The quality factor of such a structure is relatively low since it is of the order of 3, given the volume of liquid that is displaced by the structure set in motion and the significant mechanical stressing of the anchor points. Moreover, this structure is bulky because the fixed-fixed beam is in the form of a bridge under which is arranged an actuation electrode. This bulk is further increased by the presence of the detection means, in this case a laser source and a photodiode.
Also worth citing are the works regarding the use of a cantilevered beam-type electromechanical oscillator. The results of these works are explained in detail in the article by J. Teva et al., “A femtogram resolution mass sensor platform based on SOI electrostatically driven resonant cantilever. Part II: Sensor calibration and glycerine evaporation rate measurement”, Ultramicroscopy 2006, 106, 808-814. A microdroplet of glycerine is deposited accurately on this oscillator, then resonance frequency measurements are performed to follow the speed of evaporation of the droplet. This method, which consists in extracting a weight variation from a frequency measurement, entails a preliminary step for calibration of the mass sensitivity based on latex balls.
It is not possible to consider that the oscillator is placed in a liquid medium during the measurement, since only a droplet is deposited at the end of the oscillator, the rest of the detector being kept in a dry medium. The use of the device described in this article by Teva has therefore never been validated in an aqueous medium. The use of such an oscillator for biological purposes cannot therefore be envisaged because it is necessary, in this case, to extract a reaction kinetic in real time and therefore, generally, in an aqueous medium.
Finally, other works can be cited which have consisted in incorporating a biological solution to be analyzed within the oscillator itself. These works are explained in detail in the article by S. Manalis et al., “Weighing of biomolecules, single cells and single nanoparticles in fluid”, Nature 446, Apr. 26, 2007, (7139): 1066-1069.
The oscillator is in the form of a cantilevered beam in which there is provided a fluid stream tightly isolated from the medium in which the oscillator, of which it forms part, is actuated. This oscillator is set to vibrate by electrostatic coupling at 220 kHz and in a powerful vacuum, whereas the biological solution to be analyzed flows within the integrated fluid stream. In this configuration, a quality factor of 15 000 is obtained, and remains unchanging whether the channels are filled with air or with liquid. These works have made it possible to follow the reaction kinetics of an antibody-antigen recognition, for an antigen concentration of 100 ng.ml−1, which places this type of device at the same rank as the best quartz balances in terms of resolution.
These works do, however, have limitations, lying notably in the detection method employed. In practice, it implements an optical detection using a laser and an external photodiode, which contribute to the overall bulk of the system.
It emerges from this presentation that there is currently no device for gravimetric detection that implements a cost-effective NEMS/MEMS oscillator, allowing for a detection of particles in a liquid medium, in conditions that are compatible with biological analyses, offering a high quality factor and a reduced bulk.